1. Field of the Invention
The present invention relates generally to correcting measured data that is affected by errors and deviates from real data.
2. Discussion of the Background
An x-ray computed tomography (CT) imaging device is shown in FIG. 1, and the CT device includes a gantry 1 that accommodates a rotating ring 2, an x-ray source 3 that generates an x-ray cone-beam, and an x-ray filter 4. The gantry 1 has an array type x-ray detector 5 including a variety of detector elements 5A arranged in 1-D or 2-D rows. Other arrangements of the detector elements 5A of the x-ray detector 5 are also possible. FIG. 2 shows 10 rows each having 1,000 detector elements with an x-ray flux shown schematically emitted from a focal point F.
The x-ray source 3 and the array detector 5 are installed on a rotating ring 2 and face opposite sides of a subject (not shown), which lays on a sliding bed 6. Each detector element 5A of the array detector 5 corresponds to a channel. The x-ray source 3 is directed to the subject through the x-ray filter 4 and a high voltage generator 7 activates the x-ray source 3 when an x-ray controller 8 supplies a trigger signal. The high voltage generator 7 applies a high voltage to the x-ray source 3 with a timing with which the trigger signal is received. This causes the x-rays to be emitted from the x-ray source 3 and a gantry/batch controller 9 synchronously controls a revolution of the rotating ring 2 of the gantry 1 and the sliding of the sliding bed 6. A system controller 10 constitutes the control center of the entire system and controls the x-ray controller 8, the gantry/batch controller 9, the bed 6, and rotates the rotating ring 2 around a desired path around the subject while the subject is irradiated by the x-ray source 3.
The detector elements 5A of the array detector 5 are capable of measuring an intensity of the x-ray generated by the x-ray source 3, with and without the subject being interposed between the x-ray source 3 and the detector elements 5A. The detector elements 5A are also capable of measuring other characteristics of the x-ray from which an image of the subject is constructed. Thus, each detector element (channel) 5A measures at least an x-ray intensity and outputs an analog output signal corresponding to that intensity. The output signals from the channels are inputted to a data collection unit 11, which amplifies the signals for each channel and converts the signals to digital signals to produce digital projection data. The digital projection data is output from the data correction unit 11 and is fed to a processing unit 12. Based on the digital projection data corresponding to each channel, the processing unit 12 preprocesses and reconstructs an image corresponding to the subject placed on the sliding bed 6.
Conventional electronic data acquisition systems employ analog detectors or transducers coupled to analog-to-digital (A/D) converters to record physical signals in a digital format. The detectors and/or the A/Ds have a limited input parameter signal range over which they can accurately record a physical parameter of interest, for example the intensity of the x-ray beam. When the input parameter signal level is higher than a maximum signal range (maximum input level (SDMax)) of the detector or A/D, the detector or A/D output signal (measured data) can no longer accurately reproduces the input signal (real data), and the detector or A/D output “clips.” When the detector or A/D output clips, a problem appears in the background art because the signal output of the detector or A/D remains constant over time, as long as the input parameter signal has a level higher than the maximum input level.
Thus, irrespective of a change in the input parameter signal (that corresponds to a change of the physical parameter of interest), if the changed input parameter signal is still higher than the maximum input level, the signal output of the detector or A/D is constant. In other words, the detector or A/D is unable to detect the “real” value of the physical parameter of interest and instead detects the maximum input level SDMax. The maximum input level SDMax is a characteristic of device, and thus various devices will degrade the real data in different ways. The condition of the detector or A/D being unable to detect the real value of the physical parameter of interest is referred throughout this disclosure as an “overflow” condition, and this condition is applicable to other electrical units than the detector or the A/D.
The output from the detector or A/D may be processed by a number of steps in a processing chain to produce reconstruction data and a profile of the reconstruction data changes at different steps along the processing chain although the signal output of the detector or A/D is constant. The final result of the processing will be inaccurate and incorrect when the overflow condition exists. Thus, a problem appears for any device that measures an input parameter and based on that measurement creates or recreates an image of an object because the final recreated image of the object will be different under overflow conditions than the real object.
In a specific example of an x-ray CT, overflow conditions exist at least in one of the detectors or the A/Ds and these conditions cause artifacts to appear in the reconstructed images of various objects. Thus, there is a problem that the artifacts diminish the quality of the end product of the CT, blurring parts of the object examined by the CT and/or creating dark ring images. Accordingly, the results of the background art x-ray CT systems could not produce reconstructed images without ghosts and false characteristics, rendering the x-ray CT images less reliable for diagnosing various patient conditions.